1. Field of the Invention
The present invention relates to an arrangement for protecting data communication between vehicles and track conductor loops in a continuous automatic train control system.
2. Description of the Prior Art
For track conductor loops, a standards committee has recommended the following systems of laying:
The system of laying referred to as B.sub.2 consists of: conductors transposed at regular intervals and both running approximately in the middle between the rails.
The system of laying referred to as B.sub.3 consists of: conductors transposed at regular intervals one of which runs in the middle between the rails, while the other is laid along the rail between flange and web.
In both of the aforementioned systems of laying, the conductors and the rail form a three-conductor system in which characteristic impedances ZL (conductor-conductor) and ZE.sub.1, ZE.sub.2 (conductor-rail) are effective. ZE is generally considerably smaller than ZL.
To achieve undisturbed wave propagation along a track conductor loop, which can be regarded as a twin line, it is common practice to use a balanced termination of the twin line as described in German Auslegeschrift (DT-AS) 2,219,644 and in the appertaining addition, German Offenlegungsschrift (DT-OS) 2,304,733. This balanced termination consists of two series-connected component resistances whose junction is connected to rail potential directly or via a capacitance.
This simple termination considerably improves the balance of the level and phase characteristics along a track conductor loop, but is not capable of matching to the characteristic impedance, which varies with weather conditions. Especially under wet or snow conditions, standing waves thus form along the track conductor loop due to mismatching of the component resistances; if the two component resistances are mismatched differently, these standing waves occur unbalanced, i.e., they differ in phase on the two conductors. In practice, this unbalance occurs very frequently and is due to inhomogeneities of the conductors as are unavoidable at switches, for example. At the transpositions of the track conductor loops, it results in abrupt level changes and in phase shifts of the received signal different from 180.degree. which may cause errors in the information transmission and in the determination of train location.